1. Field of the Invention
The present invention relates to a porous body, a structure obtained by packing the porous body with a material, and a method for production thereof.
2. Related Background Art
As a technique for fabricating a minute porous structure on the surface of an object, a technique known as anodization has been known in the past. This technique immerses aluminum or silicon as an anode into a solution, and carries out etching through an applied electric field, to thereby produce pores in which the pore diameter is of a nanometer scale.
If an electric field is applied to an acidic electrolyte in which an aluminum substrate, for example, serving as an anode has been immersed, a porous shaped anodized film is formed from the aluminum surface. A characteristic of this porous shaped anodized film is that the film possesses a particular structure in which minute cylindrical pores (alumina nanopores) having a diameter of between a few nm and a few hundred nm are arrayed in intervals of from several tens of nm to several hundred nm. Such pores possess an extremely high aspect ratio, and have excellent cross-sectional diameter uniformity. The intervals and aspect ratio of a fabricated porous structure can be controlled by the anodization conditions. Usually, a pore interval is proportional to the applied voltage during anodization, and the pore depth is proportional to the anodization time. Further, pore diameter can be arbitrarily expanded by, after carrying out anodization, dipping alumina into a solution such as aqueous phosphoric acid which uniformly etches.
An alumina nanopore away fabricated in such a way is random. Nevertheless, a phenomenon is known wherein to a certain extent pores naturally away in an orderly manner as a result of etching for a long period of time under constant anodization conditions (type of solution, solution temperature, applied voltage, etc.) known as self-regularization conditions.
A technique which has also been devised to make a nanopore array more orderly forms a pore formation starting point, such as a recess, prior to carrying out anodization. This technique is a method (nanoimprinting process) which forms a pore by using a recess to act as a starting point through anodization, wherein a recess is fabricated on an aluminum surface as a pore formation starting point using a stamper or the like provided with a protrusion (U.S. Pat. No. 6,139,713; Masuda, “Kotai Butsuri (Japanese Journal of Applied Physics)” 31, p. 493 (1996)).
According to this technique, a stamper provided with a protrusion is pressed onto the surface of an aluminum substrate, whereby a recess is formed on the aluminum substrate surface, to thereby fabricate an anodization pore formation starting point. Therefore, compared with forming a recess directly on the aluminum surface by electron beam lithography or a similar method, although high pressure is required, a desired array of recesses that are completely orderly can be formed easily in a short period of time. By forming a desired array of recesses in advance, pores can be formed with the recesses as starting points, whereby as a result an orderly pore (nanopore) structure can be obtained in a desired array.
The fabrication of a high aspect porous body formed in an orderly manner such as this is extremely difficult using a conventional method such as photolithography, electron beam exposure, X-ray exposure or the like. In addition, since particular added functionality can be expected that differs from a random structure, this technique has been paid a great deal of attention in recent years. Furthermore, in addition to proposals for packing a metal, semiconductor or magnetic substance into such a structure, there have also been proposals made for a variety of applications, such as coloration, an EL device, an electrochromic device, a solar cell, a vacuum microdevice, a magnetic device or magnetic recording medium, a photonic device, a quantum effect device, a chemical sensor and the like.
However Japanese Patent Application Laid-Open No. H10-121292 discloses that high pressure is required if providing a recess on an aluminum substrate surface, as a stamper is being directly pressed onto the aluminum substrate. In addition, due to the high pressure, on occasion the stamper protrusion will be missing, whereby it becomes impossible to form a desired starting point.
The present invention has been created in view of such background art, wherein it is an object of the present invention to form a nanopore in a desired array without requiring high pressure.
It is also an object of the present invention to provide a more orderly porous body by having an oxidation step.